LeetCode-226. Invert Binary Tree

Invert a binary tree.

4
/   \
2     7
/ \   / \
1   3 6   9

to

4
/   \
7     2
/ \   / \
9   6 3   1

Trivia:
This problem was inspired by this
original tweet by Max
Howell:

Google: 90% of our engineers use the software you wrote (Homebrew), but you can’t invert a binary tree on a whiteboard so fuck off.

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思路分析：递归交换左右子树，返回根节点。我的第一道LeetCode题目，最近在看树的东西，顺便就做了下。看了下More Deatils，发现这个效率还是挺低的。。慢慢来吧。

/**
* Definition for a binary tree node.
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
TreeNode* invertTree(TreeNode* root) {
if( !root ) return NULL;
TreeNode* temp;
temp = root->left;
root->left = root->right;
root->right = temp;

invertTree( root->right );
invertTree( root->left );

return root;
}
};